OK, this is both extremely simple to answer and a bit complex to explain.

Simple answer: Buy cheap oil without friction modifiers and change it often!

Complex explanation:

Wet clutch setups erode in the same oil that lubricates the transmission gears.
You wouldn't break one of those fiber packs up and toss the shreds into the crankcase of a Ferrari, would you?
No way, those fibers are gritty and cause premature wear on the bearings and such.
So, while an expensive oil may be a great lubricant for transmission gears, it still gets the same fibers entrained in it and carries those fibers to the gears and bearing surfaces.
Ideally, if you had a ton of money, use the expensive oil and change it every few rides.
Most of us can't afford that, but we CAN afford to buy cheap engine oil in bulk.
Cheap single grade engine oil changed after every few rides is better for the gears and bearing surfaces than an expensive oil that's changed less frequently.

There's two other points to consider...
-The weight of the oil
-Friction modifiers

The weight of the oil can be determined simply by seeing how certain weights affect the clutch after its been warmed up.
If your clutch keeps slipping, go heavier.
If the clutch is grabby, go lighter.
You can use anything from a 60W all the way down to automatic transmission fluid (popular for ice racing and drags).

Friction modifiers are frequently found in oils designed for high mileage engines and specialty oils (Slick50 would be a friction modifier).
Generally, if it's an expensive or specialty oil, there's probably some stuff in there you don't want.
Cheap oils are cheap because they don't have that stuff added to them.
Why don't you want friction modifiers?
They bond to the clutch and (you guessed it) reduce friction...bad for something like a clutch that you don't want slipping.
ATF is specifically designed to be both a gear lubricant and a wet clutch friction multiplier (meaning it increases friction for the clutch and holds better)...so you'll be safe with any kind of ATF (remember the weight is way down, so it might cause your clutch to slip in hot temps).

That's about all there is to know about transmission oils...changing it often is more important than expensive oil changed less frequently.
Grk posted a good oil for you to start with, 10W30 Pennzoil is probably somewhere in the middle of winter riding and hot desert riding, so it should be good for most quads on an average.